Bearing device and motor with the bearing device

ABSTRACT

A bearing device includes an oil impregnated sintered bearing that rotatably holds a shaft fixed to a rotor, and a bearing boss to which inner wall the bearing is fixed. An end of the boss is sealed with a cap. Plural grooves are formed on the inner wall of the cap, so that lubricant leaked out from the bearing can be retained therein and restored to the bearing. This structure extends the service life of the bearing.

TECHNICAL FIELD

[0001] The present invention relates generally to miniature motors andbearing devices thereof, more particularly, to bearing devices suitablefor motors employed in office automation (OA) apparatuses such aspersonal computers, work stations and laser-beam printers, oraudio-video (AV) apparatuses such as Digital Versatile Disc (DVD)apparatuses and Video Cassette Recorders (VCR).

BACKGROUND ART

[0002] OA apparatuses and AV apparatuses use plural motors per apparatusas their driving sources. Those motors mainly use oil impregnatedsintered bearings because of inexpensive cost and easy handling.

[0003] Those motors employed in OA apparatuses and AV apparatuses havefrequently spun at higher speeds than conventional motors due to recentspeeding up and sophistication of the apparatuses. For instance, aspindle motor for driving a disc of a CD-ROM device mounted in apersonal computer is required to spin as fast as 10,000 rpm although aconventional motor spins at several hundreds rpm.

[0004] The speeding up and sophistication of the foregoing apparatusesremarkably increase information volume processed in electronic circuitsof the apparatuses, and the electronic circuits thus generate greaterheat amount. A blower, using a fan driven by a motor, for cooling theheat amount draws attention from the market, and a fan motor is alsorequired to spin faster.

[0005] In general, those OA and AV apparatuses are used onmaintenance-free basis, and the service life of those apparatuses oftendepends on the life of bearings of the motors used in the apparatuses. Alonger life has been demanded to the bearings in order to extend theservice life of the apparatuses.

[0006] A conventional bearing disclosed in Japanese Patent Gazette No.2903664 has been well known. FIG. 12 shows a structure of theconventional bearing device of a motor. In FIG. 12, shaft 104 fixed torotor 108 is rotatably supported with oil impregnated sintered bearing103. A first end of bearing boss 102 which firmly holds bearing 103 issealed with cap 109 to prevent lubricant 115 from leaking. At a secondend (open end) of boss 102, oil shield washer 105 is provided, andgrooves are formed on outer wall of bearing 103 as well as grooves areformed on bearing boss 102. This structure prevents lubricant 115 fromleaking out to outside of the bearing device. Stopper 106 is rigidlymounted to shaft 104 to prevent shaft 104 from coming off. Betweenstopper 106 and lower end of bearing 103, thrust plate 107 is prepared.

[0007] In the foregoing structure, however; the motor is forced toincrease its rpm due to speeding up and sophistication of the apparatus,so that the temperature of the bearing rises due to lubricant's viscousheat as well as load in the radial direction increases due to unbalanceof the rotor. As a result, the foregoing structure cannot extend themotor's life or even maintain the present rated life.

[0008] A service life of bearings are determined by the factors such asdegradation or consumption of lubricant impregnated in the oilimpregnated sintered bearing, or wear of sliding face of the bearing. Inthe foregoing conventional bearing device, the first end of the bearingis completely sealed with the cap, and lubricant leaks out from the oilimpregnated sintered bearing due to temperature rise or vibration duringthe operation and pools in the cap. The lubricant pooled cannot returninto the bearing, so that the amount of the lubricant in the bearingeventually decreases. The sliding face of the shaft thus cannot beprotected with sufficient lubricant, this is the same phenomenon as thelubricant is consumed. As a result, the service life of the bearingdevice is shortened.

DISCLOSURE OF THE INVENTION

[0009] The present invention addresses the problem discussed above, andaims to provide a bearing device that can supply lubricant within thedevice effectively to the sliding face of a shaft, thereby extending aservice life of the bearing device.

[0010] The bearing device of the present invention comprises thefollowing elements:

[0011] an oil impregnated sintered bearing rotatably supporting a shaftrigidly mounted to a rotor;

[0012] a bearing boss firmly holding the bearing with inner wall of thebearing boss; and

[0013] a cap for sealing an end of the bearing boss.

[0014] A plurality of grooves are formed on the inner wall of the capfor retaining the lubricant leaking out from the bearing and forreturning the lubricant to the bearing. This structure allows stablesupply of lubricant to the sliding face of the shaft, so that theservice life of the bearing is extended.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 shows a sectional view illustrating a structure of a fanmotor equipped with a bearing device in accordance with a firstexemplary embodiment of the present invention.

[0016]FIG. 2 shows a detailed sectional view of the bearing device shownin FIG. 1.

[0017]FIG. 3 shows a sectional view of a bearing boss of the bearingdevice shown in FIG. 2.

[0018]FIG. 4 shows a perspective view of a cap of the bearing deviceshown in FIG. 2.

[0019]FIG. 5 shows a detailed sectional view illustrating a vicinity ofthe cap of the bearing device shown in FIG. 2.

[0020]FIG. 6 shows a detailed sectional view of a bearing device inaccordance with a second exemplary embodiment.

[0021]FIG. 7 shows a detailed sectional view illustrating a vicinity ofthe cap of the bearing device shown in FIG. 6.

[0022]FIG. 8 shows a detailed sectional view of a bearing device inaccordance with a third exemplary embodiment.

[0023]FIG. 9 shows a detailed sectional view illustrating a vicinity ofthe cap of the bearing device shown in FIG. 8.

[0024]FIG. 10 shows a sectional view illustrating a structure of a fanmotor equipped with a bearing device in accordance with a fourthexemplary embodiment of the present invention.

[0025]FIG. 11 shows a detailed sectional view of the bearing device inaccordance with the fourth exemplary embodiment.

[0026]FIG. 12 shows a sectional view of a conventional motor.

EXEMPLARY EMBODIMENTS OF THE INVENTION

[0027] Exemplary embodiments of the present invention are demonstratedhereinafter with reference to the accompanying drawings.

[0028] First Exemplary Embodiment

[0029]FIG. 1 shows a sectional view illustrating a structure of a fanmotor equipped with a bearing device in accordance with the firstexemplary embodiment of the present invention. FIG. 2 shows a detailedsectional view of the bearing device of the first embodiment. FIG. 3shows a sectional view of a bearing boss of the bearing device shown inFIG. 2. FIG. 4 shows a perspective view of a cap of the bearing device.FIG. 5 shows a detailed sectional view illustrating a vicinity of thecap of the bearing device.

[0030]FIG. 1 shows a structure of a fan motor for cooling an apparatus.In FIG. 1, housing 1, an external member of the motor, and bearing boss2 are unitarily molded with resin. The stator of the motor is fixed toboss 2. The bearing device is constructed in the following way: Oilimpregnated sintered bearing 3 is press-fitted into the inner wall ofboss 2. Shaft 4 fixed to rotor 8 is inserted into bearing 3, whichrotatably supports shaft 4. Oil shield washer 5 made from resin ispress-fitted on the upper section of shaft 4, and stopper 6 made frommetal is press-fitted on the lower section of shaft 4. The lower end ofshaft 4 contacts with thrust plate 7 made from anti-abrasion resin.Thrust plate 7 is fixed with cap 9 molded with the same resin as used inhousing 1, and bears the thrust load of rotor 8.

[0031] On the outer wall of bearing boss 2, stator 10 on which coils arewound and printed circuit board 11 to which a driving circuit isprovided are rigidly mounted. On the outer wall of rotor 8, fan 12 isrigidly mounted. On the inner wall of rotor 8, magnets are fixed. Themagnets are opposite to stator 10 via annular space. The coils arepowered from outside of the motor via circuit board 11, so that rotatingtorque is generated between the magnets of rotor 8 and stator 10. Thistorque rotates fan 12 to give a blow.

[0032] Next, the bearing device in accordance with the first embodimentis further detailed with reference to FIG. 2 and FIG. 3, which show astructure of bearing boss 2 as follows: A first end of boss 2 is sealedwith cap 9 and a second end thereof is open. The upper end 13 (openside) of boss 2 has a smaller diameter than the outer diameter ofbearing 3. A plurality of grooves 14 are formed axially on the innerwall of boss 2 from its upper end 13 to just above bearing 3. The innerdiameter of bearing 3 is greater at the intermediate portion than thatof other portions, i.e. bearing 3 is an oil impregnated sintered bearingand has a middle-escaping structure. The outer diameter of the upperportion of bearing 3 is smaller than that of other portions.

[0033] The bearing device discussed above works in the following way:Rotation of shaft 4 causes a part of lubricant, which oozes because ofpressure increase due to temperature rise or vibrations, to leak towardthe rotor by travelling on shaft 4. In this case, the structure employedin the first embodiment prevents lubricant 15 from leaking to outside.To be more specific, when lubricant 15 leaking and travelling on shaft 4arrives at oil shield washer 5, centrifugal force blows lubricant 15away. Then lubricant 15 attaches to the upper portion of bearing boss 2,where grooves 14 are formed. Grooves 14 retains lubricant 15 using thesurface tension of the lubricant and prevents lubricant 15 from leakingto outside. Lubricant 15 retained in grooves 14 travels through a spacebetween bearing 3 and boss 2 due to a capillary phenomenon, then arrivesat the upper end of bearing 3. Finally lubricant 15 is absorbed intobearing 3 again.

[0034] In the lower portion of the bearing device, metallic stopper 6 ispress-fitted to the lower section of shaft 4 in order to prevent rotor 8from coming off. The lower end of shaft 4 forms like an arc, and the tipof the arc contacts with thrust plate 7, made from anti-abrasion resin,almost in a point contact status. Sliding in such a nearly point-contactstatus reduces contacting area, so that very small torque can rotateshaft 4. This structure is thus effective to reduce the powerconsumption of the motor. However, a large pressure is applied to thrustplate 7, therefore, a selection of the material of thrust plate 7 iscritical. To be more specific, PET (polyethylene terephthalate) withfiller, PEEK (polyetherethreketone), or PI(polyimide) are excellent inanti abrasion and effective to this application.

[0035] Thrust plate 4 is held with cap 9 molded with the same resin asbearing boss 2. On the inner wall of cap 9, grooves 16 are provided asshown in FIG. 4, and grooves 16 retain lubricant 15 leaked out frombearing 3. Further, cap 9 and bearing 3 are mounted such that theycontact with each other or they are close to each other, whereby thelubricant retained in grooves 16 is supplied to bearing 3 using acapillary phenomenon. Therefore, lubricant 15 does not pool at thisspace unlike a conventional one, and the lubricant within the bearingdevice can be efficiently used. As a result, the life of the bearing canbe extended.

[0036] Cap 9 is rigidly mounted in the following way: As shown in FIG.3, bearing boss 2, to which cap 9 is inserted, has flare section 17,where the inner diameter gradually becomes greater, and annular collarsection 18.

[0037]FIG. 5 details cap 9 after the motor is assembled. Cap 9 is keptcontacting with bearing 3, then collar section 18 is pressed with aheated jig, so that collar section 18 is welded and cap 9 is fixed tobearing boss 2. At this time, if cap 9 solidly contacts with bearingboss 2, the lubricant will not leak to the outside; however, it ispractically impossible to realize the complete solid contact. Therefore,in this first embodiment, the following idea is adopted to preventlubricant 15 from leaking outward using the properties of fluid even ifthe solid contact cannot be realized.

[0038] In FIG. 5, flare section 17 provided to bearing boss 2 formsenlarging space 19 between the outer wall of cap 9 and the inner wall ofboss 2, and space 19 enlarges gradually as the distance from bearing 3increases. The fluid has a property of reducing its surface area assmall as possible due to the surface tension. Thus, if lubricant 15leaks to this enlarging space 19, the surface tension produces forcethat pushes back lubricant 15 toward bearing 3, and lubricant 15scarcely leaks out. In other words, the foregoing structure works as asurface tension seal.

[0039] To maximize the performance of this surface tension seal, boss 2and cap 9 are preferably made from the same material, or the surfaces ofboth boss 2 and cap 9 are coated with the same material, so thataffinities of both of the surfaces and that of lubricant 15 can bematched. If the affinities between both of boss 2, cap 9 and lubricant15 are extraordinarily different from each other, the sealing effectsometimes does not work well.

[0040] In the foregoing description, flare section 17 is provided to theinner wall of boss 2; however, the flare section can be prepared on theouter wall of cap 9. This structure produces, not to mention, a similaradvantage to what is discussed above.

[0041] In this first embodiment, the structure discussed above allowslubricant 15 once leaked out from bearing 3 to return into bearing 3.Thus the lubricant can be supplied well to the sliding face of shaft 4.Since this structure prevents the lubricant from leaking to the outside,the life of the bearing device can be extended.

[0042] In this first embodiment, bearing boss 2 and cap 9 are fixed toeach other by heat welding; however, other methods such as ultrasonicwelding, press-fitting or bonding can produce a similar advantage. Inthe case of ultrasonic welding, a jig in small oscillation is applied tothe resin to melt locally with frictional heat between the jig and theresin. In the case of press-fitting, cap 9 is sometimes deformed whichdegrades the vertical of thrust plate 7 with respect to shaft 4, and inthe case of bonding, the adhesive sometimes chemically reacts with thelubricant to degrade the lubricant. Thus the heat welding or theultrasonic welding is preferable to this structure.

[0043] Second Exemplary Embodiment

[0044]FIG. 6 shows a detailed sectional view of a bearing device inaccordance with the second exemplary embodiment. FIG. 7 shows a detailedsectional view illustrating a vicinity of the cap of the bearing deviceshown in FIG. 6.

[0045] In the second embodiment, the lubricant sealing mechanism and theretaining mechanism of the cap are further improved. In the previousfirst embodiment, cylindrical cap 9 is used; however, as shown in FIG. 6and FIG. 7, cap 20 in accordance with the second embodiment has brim 21at the lower section, and brim 21 has a greater outer diameter than thatof other parts of cap 20. The outer wall of cap 20 is fixed to the innerwall of bearing boss 2. Flare section 22 of boss 2 has a greater flaringangle than that in the first embodiment, so that space 23 formed betweencap 20 and boss 2 is greater than enlarging space 19 in the firstembodiment.

[0046] Other structural points such as grooves 16 are provided on theinner wall of cap 20 for restoring the lubricant, and cap 20 is fixed atits brim by heat welding, are similar to those in the first embodiment.

[0047] Brim 21 provided to cap 20 reduces a gap (or increases adhesion)between cap 20 and welded section when cap 20 is fixed to boss 2 at thebrim by heat welding. Therefore, if force is applied in the direction ofcoming cap 20 off, stress to the welded section is smaller than thefirst embodiment. As a result, higher strength of the structure can beobtained than the first embodiment.

[0048] Since space 23 is greater than that of the first embodiment, thesurface tension of the lubricant in space 23 is stronger, which canfurther restrain the lubricant from leaking. Since space 23 has a largercapacity, if the lubricant leaks to space 23 due to vibrations orshocks, the lubricant is once pooled in space 23 before being pushedback into the bearing. In other words, this structure further preventsthe lubricant from leaking to the outside.

[0049] As discussed above, the second embodiment can provide ahigh-performance motor that is stronger in structure and has lesspossibility of lubricant leakage than the first embodiment. And yet,this high-performance motor can be manufactured with the same number ofparts and the same manufacturing method as the first embodiment althoughthe shape of cap 20 is rather complicated than that of the firstembodiment.

[0050] Third Exemplary Embodiment

[0051]FIG. 8 shows a detailed sectional view of a bearing device inaccordance with the third exemplary embodiment. FIG. 9 shows a detailedsectional view illustrating a vicinity of the cap of the bearing deviceshown in FIG. 8.

[0052] In this third embodiment, a first end of bearing boss 24 is fixedby cap 25 in a similar manner to the first and the second embodiments.However, cap 25 in this third embodiment is shaped like a simple disc,and does not have grooves for restoring lubricant. A plurality ofgrooves 26 are formed, instead, on the inner wall of bearing boss 24 atthe cap side.

[0053] In the first and second embodiments, the cap is made from thesame resin as that of the housing; however, cap 25 in the thirdembodiment is made by pressing a plated steel plate. In general, metalstamping can form products at a higher speed and at a lower cost thaninjection molding of resin although the stamping can only handle simpleshapes. Cap 25 in the third embodiment is thus less expensive than thosein the first and second embodiments.

[0054] In FIG. 9, bearing boss 24 is equipped with a step by molding onthe inner wall at its lower portion. Cap 25 is press fitted into thelower portion (having a larger diameter) of bearing boss 24, and theouter wall of cap 25 is sealed by adhesive 27 to prevent the lubricantfrom leaking. This structure makes cap 25 be subjected to strong stress,and cap 25 thus tends to be deformed. Therefore, cap 25 is made ofplated steel plate that is stronger than resin. This structure restrainscap 25 from being deformed.

[0055] Grooves 26, for retaining and restoring the lubricant, are formedon the inner wall of bearing boss 24 at a smaller diameter side andextend until the section overlaps with the lower section of bearing 3.Overlapping grooves 26 with bearing 3 makes these two elements face toeach other in a wider area, so that the lubricant can be restoredsmoothly.

[0056] Grooves 26 become shallower as they approach the bearing, and theinner diameter of the grooved wall is slightly greater by several toseveral tens μm than that of the inner wall where bearing 3 is fixed.

[0057] The difference in two inner diameters, i.e., an inner diameter ofthe section where a depth of grooves 26 varies and an inner diameter ofthe inner wall where bearing 3 is fixed or held by the peaks of grooves26, allows placing inner diameter pins of the metal die with ease whenthe die is produced. The difference also improves releasing of theproduct from the die. The grooves shallower section produces force thatpushes back the lubricant into bearing 3 due to the surface tension, sothat grooves 26 increase their restoring effect of the lubricant. Thedifference in the inner diameters at the inner wall of bearing boss 25guides bearing 3 to be press-fitted into boss 24, so that the outer wallof bearing 3 and the inner wall of boss 24 are protected againstscratches and bearing 3 is prevented from slanting.

[0058] Further, as shown in FIG. 8, bearing 3 is firmly held using onlyits outer wall in part where escape is provided at its inner wall, sothat holding strength of bearing 3 weakens; however, this structurerestrains the contraction of the inner diameter of the bearing atpress-fitting into boss 24. Thus the accuracy of the inner diameter ofbearing 3 increases.

[0059] The advantages discussed above allow the third embodiment toprovide a long-life motor similar to the first and second embodiments.

[0060] Fourth Exemplary Embodiment

[0061]FIG. 10 shows a sectional view illustrating a structure of a fanmotor equipped with a bearing device in accordance with the fourthexemplary embodiment of the present invention. FIG. 11 shows a detailedsectional view of the bearing device in accordance with the fourthexemplary embodiment.

[0062] The fourth embodiment provides a different structure from thoseof the previous embodiments, and it can produce similar advantages ormore than similar advantages than those of the previous embodiments.

[0063] A structure of the fan motor in accordance with the fourthembodiment is almost the same as that shown in FIG. 1 of the firstembodiment. Only the difference is a construction around cap 28. To bemore specific, cap 9 in accordance with the first embodiment is madefrom the same resin as bearing boss 2; however, cap 28 used in fourthembodiment is made from porous sintered alloy. The flare section formedat the bearing boss is eliminated in the fourth embodiment.

[0064] In this fourth embodiment, the sintered alloy forming cap 28 hasintentionally a greater hole diameter than that of the sintered alloyused in oil impregnated sintered bearing 3. When cap 28 is brought intocontact with bearing 3, this difference in hole diameters can supplylubricant 15, soaking into cap 28 due to a capillary phenomenon, tobearing 3.

[0065] This structure absorbs lubricant 15, leaked not only to the innerwall of cap 28 but also to the bottom of cap 28 and the outer wallcontacting with bearing boss 2, without any loss and returns it tobearing 3. As a result, the life of the bearing can be further extended.Cap 28 can soak up in advance the same lubricant as the oil that isimpregnated into sintered bearing, so that a total amount of thelubricant in the whole bearing device increases, which further extendsthe bearing life. A void content of the oil impregnated sintered bearingis set, in general, at a value ranging from 10 vol % to 30 vol %. Thisvalue does not adversely affect the supply of lubricant to the slidingface or an oil film pressure of the shaft sliding face. The material ofcap 28 in the fourth embodiment preferably has a volume void content aslarge as possible unless the strength or oil retaining ability of cap 28is adversely affected. A greater void content can increase the amount ofthe lubricant, so that the bearing life can be further extended.

[0066] In the foregoing description, cap 28 made from porous sinteredalloy is taken as an example; however, other porous materials such asporous resin can produce a similar advantage.

[0067] Although the previous embodiments show that the bearing standsvertically, the lubricant restoration to the sliding face of the bearingdue to the capillary phenomenon, or the sealing effect due to thesurface tension of the lubricant can be produced regardless of anorientation of the bearing. The previous embodiments use a fan motor asan example; however, other motors using similar bearing devices canextend their service lives.

INDUSTRIAL APPLICABILITY

[0068] A bearing device includes an oil impregnated sintered bearingthat rotatably holds a shaft fixed to a rotor, a bearing boss whichfirmly holds the bearing with its inner wall, and a cap that seals anend of the boss. A plurality of grooves are formed on the inner wall ofthe cap, so that lubricant leaked out from the bearing is retainedtherein and returned to the bearing. This bearing device can be used inminiature motors employed in OA apparatuses or AV apparatuses and extendthe life of the bearing.

1. A bearing device comprising: an oil impregnated sintered bearing thatrotatably holds a shaft fixed to a rotor; a bearing boss firmly holdingthe bearing with inner wall thereof; and a cap for sealing one end ofthe bearing boss, wherein a plurality of grooves are formed on innerwall of the cap for retaining lubricant leaked out from the bearing andfor restoring the lubricant to the bearing.
 2. The bearing device ofclaim 1, wherein at least one of a part of the boss and a part of thecap forms a flare section, so that a space is provided between the bossand the cap, and the space enlarges as a distance from the bearingincreases.
 3. The bearing device of claim 1, wherein the cap and thebearing boss are made from an identical material.
 4. The bearing deviceof claim 1, wherein the cap and the inner wall of the bearing boss arecoated with an identical material.
 5. The bearing device of claim 1,wherein a part of inner wall of the bearing boss forms a flare sectionand a brim is formed on outer wall of the cap, so that a space isprovided between the bearing boss and the cap, and the space enlarges asa distance from the bearing increases.
 6. A bearing device comprising:an oil impregnated sintered bearing that rotatably holds a shaft fixedto a rotor; a bearing boss firmly holding the bearing with inner wallthereof; and a cap for sealing one end of the bearing boss, wherein aplurality of grooves are formed on inner wall of the bearing boss at thecap side for retaining lubricant leaked out from the bearing and forrestoring the lubricant to the bearing.
 7. The bearing device of claim6, wherein a closer section to the bearing of the grooves becomesshallower.
 8. The bearing device of claim 6, wherein the grooves axiallyoverlap with a lower section of the bearing.
 9. The bearing device ofclaim 6, wherein an inner diameter at a section, where the grooves areformed, of the bearing boss is slightly greater than the inner diameterat a section, where the bearing is firmly held, of the bearing boss. 10.A bearing device comprising: an oil impregnated sintered bearing thatrotatably holds a shaft fixed to a rotor; a bearing boss firmly holdingthe bearing with inner wall thereof; and a cap for sealing one end ofthe bearing boss, wherein the cap is made of porous material forretaining lubricant leaked out from the bearing and for restoring thelubricant to the bearing.
 11. The bearing device of any one of claim 1,6, or 10, wherein the cap is brought into contact with the bearing. 12.The bearing device of claim 10, wherein a hole diameter of the porousmaterial is greater than that of material of the bearing.
 13. Thebearing device of claim 10, wherein a volume void content of thematerial of the cap is greater than that of material of the bearing. 14.A motor equipped with the bearing device as defined in any one of claims1 through 13.